The present invention generally relates to the field of diagnostic imaging and, more specifically, to an array of electroacoustic transducers and an electronic probe for diagnostic imaging with a high focusing depth.
Transducer arrays are widely used for making ultrasound probes and they are the device for generating acoustic radiation beams or the device for receiving acoustic signals and for converting them into electric signals. Generally the same transducer array is used alternately both for generating acoustic radiation beams to be transmitted and for receiving acoustic pulses to be converted into electric signals. However the arrangement with an ultrasound probe provided with two transducer arrays operating independently one of which for transmitting acoustic radiation beams and the other one for receiving acoustic pulses cannot be ruled out.
As regards conventional ultrasound probes, each transducer element is composed of an electroacoustic element, for example a piezoelectric one, an electrode for the input/output of an electric signal exciting the electroacoustic element corresponding to the emission of an acoustic signal and an electric reception signal corresponding to an acoustic signal impinging on the corresponding electroacoustic element respectively being associated thereto, each electrode being in turn connected to a dedicated line for transmitting the electric excitation signal or the electric reception signal respectively and moreover each electroacoustic element being connected to a ground electrode and said electroacoustic elements being backed by an array made of acoustically and electrically insulating material wherein they are at least partially embedded the transducer array being provided with a predetermined number of transducers.
As regards such conventional probes, transducer elements composing the array of transducer elements are used alternately both for generating and emitting acoustic pulses, and for receiving acoustic pulses and for converting them into reception electric signals. Therefore transducer arrays are intended for generating the acoustic radiation beam that is transmitted to a body under examination and also for receiving acoustic pulses from the body under examination which derive from the acoustic radiation beam previously transmitted to such body under examination being reflected.
As regards conventional ultrasound probes, for example, the transmitting/receiving head comprises a front side from which acoustic radiation ultrasound beams are emitted in a direction of propagation towards a body under examination and on which front side pulses reflected from the body under examination impinge. Said head has a back side opposite to the front side and it is oriented towards the inside of the casing of the ultrasound probe and towards means for supporting said head inside the casing.
Acoustic radiation beams are composed of acoustic pulses emitted by the individual transducer elements that are combined together such to generate an acoustic radiation beam having a predetermined direction of propagation and a predetermined focusing along said direction of propagation.
Said transmitting/receiving head generally comprises, with an order starting from the back side towards the front side and corresponding to the direction of propagation of the acoustic waves, a first layer composed of an array of contact electrodes, having each one a separate electric line for the connection to an electric contact pin being a part of a multi-pin electric connector and provided at one peripheral edge of the layer of contact electrodes. The layer composed of the array of contact electrodes is overlapped by a further layer composed of an array of electroacoustic elements, particularly piezoelectric ones. These can be composed of ceramic elements and they constitute the individual transducers converting electric excitation signals into acoustic pulses emitted from one surface thereof and/or converting acoustic pulses impinging thereon into electric signals. Each one of the electroacoustic elements of the array is coincident with a contact electrode and is electrically connected thereto for example by means of a simple surface contact of each individual contact electrode of a corresponding electroacoustic element. The array of contact electrodes and of the overlapping electroacoustic elements is backed by an acoustically and electrically insulating material that can be a simple backing layer and/or it can embed at least partially the electrodes and the electroacoustic elements filling at least for a portion of the thickness of the overlapping contact electrodes and electroacoustic elements the gaps therebetween. A third layer is composed of a ground electrode. It can be in the form of a continuous sheet overlapping the side of the array of piezoelectric elements opposite to the one overlapped by the contact electrodes. As an alternative said third layer can be made like contact electrodes by an array of individual elements which are electrically separated one from the other and each one overlapping and being electrically connected only to one of the electroacoustic elements.
Generally on the third grounding layer there are provided one or more acoustic matching layers acting for matching the acoustic impedance of the transducers to the acoustic impedance of the operation environment, for example of the body under examination in this case where the transmitting/receiving head is used within an ultrasound probe.
These conventional probes provide a transmission and reception switch which, after each excitation of the transducer elements by the electric excitation signals, connects the connection lines of the individual transducer elements to a section receiving and processing the reception signals of the individual transducer elements generated from the acoustic pulses impinging on the sensitive surfaces thereof. The receiving and processing section extract information from reception signals, for example image data.
While as regards electric excitation signals, these are generated by a unit allowing also the power of such signals to be adjusted, as regards the reception signals, their power or intensity is limited to the characteristics of the transducer elements and since the transducer array of the probe is connected to the receiving and processing unit by a relatively long cable having a certain capacitance, it is necessary to provide each transducer element with a preamplifier for the reception signal. Due to that, the reception signal is not affected by the charge constituted by the capacitance of the connection cable and therefore the sensitivity and/or the bandwidth is improved.
By using the same transducer elements both for the transmission and reception and so by using the same connection lines for transmitting the excitation signal and for collecting the reception signals, each preamplifier is provided with a decoupling circuit avoiding short-circuit conditions between the output and the input when the transducer element is under the excitation phase.
As it is clear from the above, and particularly as far as linear or convex probes are concerned, where transducer elements are arranged side by side and are spaced apart at least along a row, the length of said row corresponds to the length of the transducer array and in the art it is the so-called aperture of the transducer array. Given a predetermined aperture, each transducer element is provided with a predetermined size in the length-wise direction of said row and such size, as far as linear or convex probes are concerned where only one row of transducer elements is provided, corresponds to the width of the transducer element and it is the so-called pitch of the transducer element.
Dimensional characteristics related to both the aperture of the array of transducer elements and to the pitch of the individual transducer elements affect the characteristics about the possibility of generating acoustic radiation beams having a good focusing effect even at deep penetration depths of the beam, i.e. at relatively deep distances from the emitting surface of the transducer array and so the possibility of increasing or keeping the resolution high even at such relatively deep depths; the possibility of steering said beam, i.e. of forming the acoustic radiation beam in a direction of propagation different than the one perpendicular to the emitting surface of the array of transducer elements, the sensitivity of the transducer array as regards reflected acoustic signals that are detected and converted into reception electric signals by said transducer array.
The equation defining the position of the natural focus within a transducer or a transducer array having a length D (defined as the aperture of the transducer) is the following:F=D2/4λ  (1)
where λ is the wavelength.
Therefore the greater the length of a transducer element row is i.e. a linear or convex probe, that is the aperture of said transducer array, and the deeper the natural focus is and therefore the higher the ultrasound diagnostic image resolution is which is obtained from signals generated from said transducer array since the beam can be deeply focused reducing the size thereof. The need of making transducer arrays with apertures as wide as possible arise therefrom obviously whether the radiation lobe of the element allow them to be used. Given the number of transducers N in a transducer array (for example 192) and the scanning width L (for example 4-5 cm) the pitch (L/N) is automatically achieved that is the size of each transducer element in the direction parallel to the length-wise direction of the transducer element row forming the transducer array with the predetermined aperture D.
Each transducer element in turn has a radiation pattern which tends to diverge, with respect to the axis perpendicular to the surface emitting/receiving the acoustic pulses (direction of propagation or incidence of the acoustic pulses), by an angle θ such that:sin θ=0.6λ/a  (2)where a denotes the radius of the transducer element assuming it has a circular section, or the size of the transducer element in the direction parallel to the length of the row of transducer elements.
With reference to the formula (2) it can be deduced that the larger the pitch of the transducer element is, the less the radiation diverges and, therefore, the more the radiation beam emitted from each individual element tends to be a tube with a diameter equal to the diameter of the transducer element. Vice versa, if the transducer element tends to approximate a point source, or a narrow source, the emitted beam tends to become wider till taking a radiation pattern that is theoretically spherical or cylindrical respectively.
Moreover the more the pitch of the individual transducer element is reduced, i.e. the narrower the transducer element is, the greater the possible steering effect can be.
The above theory is described in more details in the following publication “Physics and Instrumentation of Diagnostic Medical Ultrasound” by Peter Fish, John Wiley & Sons, chapter 4, pages 27-49.
From the above it is clear that the greatest limits in increasing the acquisition resolution, particularly in the case of linear and convex probes, are due to the radiation lobe of the individual element which is too narrow (about 15 to 20 degrees on average) since currently elements are wide i.e. they have a relatively large pitch, for scanning widths to be large enough.
With reference to conventional transducer arrays, i.e. having a limited number of transducer elements and so of channels connecting them to the units generating the excitation signals and to the units processing the reception signals the fact of decreasing the pitch of the transducer elements leads to a reduction of the overall surface receiving the acoustic pulses, i.e. of the surface sensitive to said acoustic pulses and, even in the case of arrays having 192 transducer elements, the scanning width L would be small and also the far-field focusing effect and sensitivity would be poor since the maximum aperture D would be limited by L.